For the depalletization of product layers, that is to say, for picking up a layer of products from stacks of layers placed on pallets, it is known to use layer grippers which operate on the principle of suction or vacuum creation. These grippers are provided with side bars at the four sides, which close on the four sides of the product layer, thus tightening and stabilizing it. The gripper has vacuum generating means, adjustable in height for adjusting to the height of the products forming the layer.
DE 4206038 A1 shows an example of such a depalletizing gripper, in which the vacuum generating gripping means are, for example, a suction plate of the suction cup type, and also comprising a lower shutter support surface, which closes for greater safety under the layer after it has been lifted and before its handling.
Another example is given by document US 2010/0014954 A1, which shows a gripper, provided with a lower shutter support surface, which can be provided with side bars only, which close on the four sides of the product layer, thus tightening it, or also vacuum generating gripping means, consisting of a suction plate.
Other known vacuum generating means may consist of a plate-like element, preferably grid-like, which rests onto the product layer and the height whereof is adjusted according to the height of the products being processed. Through the plate-like element a vacuum with a low head but a high flow rate is made for lifting the product layer. Between this plate-like element and the side bars there are flexible curtain elements which, when the vacuum is made in the plate-like element, tend to adhere to the side walls of the product layer, thus increasing the degree of vacuum as they reduce air leakage on the side of the product. In practice, a sort of vacuum bell is generated around the product layer which is efficient for lifting the product layer even in the presence of products difficult to handle with normal suction cups, such as for example PET bottle bundles.
Document WO 00/64790 A1 shows such a gripper in which on each of the four sides, the side bars consist of inflatable elements placed between a fixed frame and flexible peripheral curtains. The side bars act in particular on such flexible curtains so as to exert pressure on the sides of the product layer.
Similarly, document EP 2593387 A1 shows a similar gripper in which the side bars consist of pressure elements, which push flexible curtain peripheral elements in contact with the side walls of the products forming a layer. A lower shutter support surface associated with the gripper support frame closes for greater safety under the layer after it has been lifted and before its handling.
However, the flexible curtain elements arranged on the perimeter of the gripper have some technical drawbacks, including in particular the reduced tolerance on the minimum or maximum dimensions of the layer, in plan and/or height. In fact, the flexible curtain elements cannot be excessively extended as they would form too many wrinkles at the corners for very small layers, but they must still be sufficiently large to cover the layer of products.
Moreover, they are subject to a more accentuated wear compared to the other mechanical components of the gripper, intrinsically linked to their flexible nature and to being subjected, during repeated clamping cycles of the gripper, to bending, pulling and repeated folds, especially in the corners where the pull on the curtain is not uniform and especially if the size of the layer is far from the expected size. They therefore require dedicated maintenance to maintain constant efficiency over time.
Flexible curtain elements also require the provision of ropes or winders or tensioners to keep them in position, which represent elements of system fragility.